I looked at this patch after it landed in 'master'. Thanks for contributing it.

A couple of minor improvements. First, the implementation of logcount can use 
the count_one_bits functions already used by Emacs, rather than reinventing that 
wheel; these functions should be just as fast as __builtin_popcount etc. when 
using GCC. Second, logcount should count zero bits in negative numbers, for 
compatibility with Common Lisp. I installed the attached two patches to 
implement these improvements.